Researching "Customary International Law" can seem like an elusive process. According to International Legal Research in a Nutshell, the basic outline for conducting such research might take a form such as this:
Hoffman, M., & Berring, R. C. (2017), pp. 72-73. International legal research in a nutshell. St. Paul, MN: West Academic Publishing.
This tab focuses on sources needed to locate materials known as "State Practice."
Yearbooks are available from HeinOnline for a variety of different countries and regions. This could be a good start when reviewing national laws and cases on issues of public international law.